The utilization of high-frequency switching circuits in power conversion applications to produce high efficiency circuits has introduced the new issue of managing electromagnetic interference produced by high-frequency waveforms. These waveforms are coupled through parasitic circuit elements, such as stray capacitances and incidental magnetic flux linkages, to other portions of a circuit, and result in electromagnetic radiation and in signal components conducted to power source and load circuits. The allowable level of EMI produced in an end product is generally regulated in the U.S. by 47 C.F.R. §15 (also referred to as “FCC Part 15”), and by other national standards in other countries.
In automotive applications, three-phase brushless motors are often employed to operate a mechanical device when a high actuator power density is required. Three-phase brushless motors can provide accurate speed control, for example, for applications such as electric power steering. The motor speed control arrangement generally employs pulse-width modulation to control the input power to the motor, with a pulse repetition frequency that is typically about 20 kHz. A drawback of conventional pulse-generation arrangements wherein drive signals are produced for the pulse-width modulated pulses that control the input power to the motor is that high current pulses coupled to the motor combined with a high pulse switching speed generally produce a high level of conducted (and radiated) EMI.
A half-bridge driver employing conventional design practices produces an output pulse-width modulated waveform with a waveform characteristic such as a rise time or a fall time that is substantially independent of an operating condition of the driven bridge. As a result, a conventional design of a half-bridge driver unnecessarily produces an unnecessarily high level of EMI, for example, at a light load on a motor. A conventional half-bridge driver arrangement may adaptively alter a pulse width in response to a sensed motor load, but an altered pulse width may be insufficient to reduce EMI to an acceptable level without an added EMI filter, such as an inductor-capacitor low-pass filter that adds unnecessary volume and cost to a product design.
Thus, there is a need for a process and related method to reduce a level of EMI produced by a power switching arrangement coupled to a driver that avoids disadvantages of conventional approaches.